le dung csmaca

8/2/2019 Le Dung Csmaca

1/31

CarrierSenseMultipleAccesswithCollisionAvoidance


2/31


3/31

CSMA/CDvs.CSMA/CA(1)

CSMA/CD(CollisionDetection)istheMACmethodusedinawiredLAN(Ethernet).WiredLANstationscan(whereaswirelessstationscannot)detectcollisions.

BasicCSMA/CDoperation:

1) Waitforfreemedium2) Transmitframe3)

Ifcollision,stoptransmissionimmediately

4) Retransmitafterrandomtime(backoff)

CSMA/CDrule:Backoffaftercollision

1.BasicCSMA/CAoperation


4/31

CSMA/CDvs.CSMA/CA(2)

CSMA/CA(CollisionAvoidance)istheMACmethodusedinawirelessLAN.Wirelessstationscannotdetectcollisions(i.e.thewholepacketswillbe

transmittedanyway).BasicCSMA/CAoperation:

1) Waitforfreemedium2) Waitarandomtime(backoff)3) Transmitframe4) Ifcollision,thestationsdonotnoticeit5) Collision=>erroneousframe=>noACKreturned

CSMA/CArule:Backoffbeforecollision



5/31

Basicwirelessmediumaccess

.HereisInfrastructurebasicserviceset(BSS).

.AllstationsandAPhaveequalpriority

transmissionindownlink(fromtheAP)anduplink(fromastation)issimilar.

CSMA:Onepacketatatime

AP

wiredLAN



6/31

Wirelessmediumaccess(1)

DIFS SIFS

ACK

(B=>A)

Transmittedframe(A=>B)

Whenaframeisreceivedwithoutbiterrors,thereceivingstation(B)sendsanAcknowledgement(ACK)framebacktothetransmitting

station(A).

Ifthereceivedframeiserroneous,noACKwillbesent

CyclicRedundancyCheck(CRC)isusedforerrordetection



7/31


DIFS SIFS DIFS

ACK

(B=>A)


Duringthetransmissionsequence(Frame+SIFS+ACK)themedium(radiochannel)isreserved.Thenextframecanbetransmittedat

earliestafterthenextDIFSperiod.

Nextframe(fromanystation)

Earliestallowedtransmissiontimeofnextframe



8/31


DIFS SIFS DIFS

ACK

(B=>A)


Therearetwomechanismsforreservingthechannel:

PhysicalcarriersensingVirtualcarriersensingusingtheNetworkAllocationVector(NAV).

Nextframe



9/31


10/31


ThetwomostimportantinterframespacingtimesareSIFSandDIFS:

SIFS(ShortInterframeSpace)=10s(16s)

DIFS(DCFInterframeSpace)=50s(34s)

Whentwostationstrytoaccessthemediumatthesametime,theonethathastowaitforthetimeSIFSwinsovertheonethathastowaitforthetimeDIFS.Inotherwords,SIFShashigherpriorityoverDIFS.

802.11b 802.11g



11/31


DIFS SIFS DIFS

ACK

Transmittedframe

NAV

Virtualcarriersensing

ItmeansthataNAVvalueissetinallstationsthatwereabletoreceiveatransmittedframeandwereabletoreadtheNAVvalueinthisframe.

NAVvalueisgivenhere Nextframe

TransmissionisnotallowedaslongasNAVisnon-zero



12/31


DIFS SIFS

Transmittedframe

NAV

VirtualcarriersensingusingNAVisimportantinsituationswherethechannelshouldbereservedforalongertime(RTS/CTSusage,

fragmentation,etc.).

ACKACK

Longtransaction

DIFS



13/31

NAVvalueiscarriedinMACheader

MPDU(MACProtocolDataUnit)

MACpayloadAddr1 Addr2 Addr3 Addr4(optional)

FCS

Durationfield:15bitscontaintheNAVvalueinnumberofmicroseconds.Thelast(sixteenth)bitiszero.

AllstationsmustmonitortheheadersofallframestheyreceiveandstoretheNAVvalueinacounter.Thecounterdecrementsin

stepsofonemicrosecond.Whenthecounterreacheszero,thechannelisavailableagain.



14/31


DIFS SIFS t>DIFS

ACK

(B=>A)


Whenastationwantstosendaframeandthechannelhasbeenidleforatime>DIFS(countedfromthemomentthestationfirstprobed

thechannel)=>cansendimmediately.

Nextframe(fromanystation)

ChannelwasidleatleastDIFSseconds



15/31


DIFS SIFS DIFS

ACK

(B=>A)


Whenastationwantstosendaframeandthechannelisbusy=>thestationmustwaitabackofftimebeforeitisallowedtotransmitthe

frame.Reason?Nexttwoslides

Nextframe

Channelwasbusywhenstationwantedtosendframe

Backoff



16/31

Nobackoff=>collisioniscertain

Supposethatseveralstations(BandCinthefigure)arewaitingtoaccessthewirelessmedium.

Whenthechannelbecomesidle,thesestationsstartsendingtheirpacketsatthesametime=>collision!

StationA

StationB

StationC

DIFS

Collision!

ACK



17/31

Backoff=>collisionprobabilityisreduced

Contendingstationsgeneraterandombackoffvaluesbn.Backoffcounterscountdownwards,startingfrombn.Whenacounterreacheszero,thestationisallowedtosenditsframe.Allothercountersstopcountinguntilthechannelbecomesidleagain.

StationA

StationB

StationC

DIFS

bnislarge

bnissmall

Backoff

RemainingbackofftimeACK



18/31

Contentionwindow(CW)for802.11b

Iftransmissionofaframewasunsuccessfulandtheframeisallowedtoberetransmitted,beforeeachretransmissiontheContentionWindow(CW)fromwhichbnischosenisincreased.

DIFS CW=25-1=31slots(slot=20

s)Initialattempt

DIFS

CW=26-1=63slots1stretransm.

DIFS CW=2

10

-1=1023slots5th(andfurther)retransmissions

:

CW 802.11b



19/31

Contentionwindow(CW)for802.11g

Inthecaseof802.11goperation,theinitialCWlengthis15slots.Theslotdurationis9s.Thebackoffoperationof802.11gissubstantiallyfasterthanthatof802.11b.

DIFS CW=24-1=15slots(slot=9

s)Initialattempt

DIFS

CW=25-1=31slots1stretransm.

DIFS CW=2

10

-1=1023slots6th(andfurther)retransmissions

:

CW 802.11g



20/31


21/31


22/31


StationA

StationB

StationC

StationD

DIFS DIFS

Defer

3)StationDalsowishestosendapacket

4)However,stationBiswinnerandstartssendingpacket3

4

ACK

2.Wirelessmediumaccessexample


23/31


StationA

StationB

StationC

StationD

DIFS

5)StationDstartssendingpacket.Nowthereisnocompetition.

DIFS

5

ACK

2.Wirelessmediumaccessexample


24/31


25/31

3.UsageofRTS&CTS

TheRTS/CTS(Request/ClearToSend)schemeisusedasacountermeasureagainstthehiddennodeproblem:

AP

WS1

WS2

Hiddennodeproblem:WS1andWS2canheartheAPbutnoteachother

=>

IfWS1sendsapacket,WS2doesnotnoticethis(andviceversa)=>collision!


26/31

ReservationofmediumusingNAV

RTS

SIFS

DIFS

NAV=CTS+Data+ACK+3xSIFS

CTS

Dataframe

ACK

SIFS

SIFS

WS1

AP

NAV=Data+ACK+3xSIFS

NAVinRTS

NAVinCTS

3.UsageofRTS&CTS


27/31

DangerofcollisiononlyduringRTS

WS2doesnotheartheRTSframe(andassociatedNAV),butcanheartheCTSframe(andassociatedNAV).

RTS

NAV=CTS+Data+ACK+3xSIFS

CTSDataframe

ACKWS1AP

NAV=Data+ACK+3xSIFS

NAVinRTS

NAVinCTS

Dangerofcollision

3.UsageofRTS&CTS


28/31

AdvantageofRTS&CTS(1)

UsageofRTS/CTSoffersanadvantageifthedataframeisverylongcomparedtotheRTSframe:

RTSCTS

DataframeACK

WS1AP

Shortinterval:collisionnotlikely

Dataframe

ACK

WS1

AP

Longinterval:collisionlikely

(RTS/CTSnotused)

(RTS/CTSused)

3.UsageofRTS&CTS


29/31

AdvantageofRTS&CTS(2)

Alongcollisiondangerinterval(previousslide)shouldbeavoidedforthefollowingreasons:

Largerprobabilityofcollision

Greaterwasteofcapacityifacollisionoccursandtheframehastoberetransmitted.

Athresholdparameter(dot11RTSThreshold)canbesetinthewirelessstation.FramesshorterthanthisvaluewillbetransmittedwithoutusingRTS/CTS.

3.UsageofRTS&CTS


30/31

4.Fragmentation

FragmentationmakesuseoftheRTS/CTSschemeandtheNAV

mechanism:

RTS

SIFS

DIFS

RTS

CTS

Frag0

ACK0

SIFS

SIFS

WS1

AP

CTS

NAVinWS

NAVinAP

Frag1

ACK1

SIFS

SIFS

Frag0

ACK0

Athresholdparameter(dot11FragmentationThreshold)canbesetinthewirelessstation.Frameslongerthanthisvaluewillbetransmittedusingfragmentation.


31/31

Sequencecontrolfield

MPDU(MACProtocolDataUnit)

MACpayloadAddr1 Addr2 Addr3 Addr4(optional)

FCS

Fragmentnumber(for

identifyingfragments)

Framesequencenumber(for

identifyingframes)

4.Fragmentation

le dung csmaca

Documents